A prior art spiral chiller 2 used for cooling and freezing food products is schematically illustrated in FIG. 1. The prior art spiral chiller 2 comprises: a housing 4 having a lower product chilling chamber 6 and an upper circulation and cooling chamber 8; a spiral conveyor belt 10 within the product chilling chamber 6 for carrying the food product during the cooling process; a chamber ceiling 12 which separates the product chilling chamber 6 from the upper circulation and cooling chamber 8; an upper side opening 11 in a sidewall of the product chilling chamber 6 through which the upper end (typically an outfeed section) 13 of the conveyor 10 extends; a lower sidewall opening 15 through which the lower end (typically an infeed section) 17 of the spiral conveyor 10 extends; a circulation fan 14 for circulating a cooling medium (e.g., air) 5 through the spiral chiller 2; an exterior fan motor 19; one or more indirect heat exchangers, refrigerating elements, or other cooling devices 21 within the upper circulation and cooling chamber 8 for cooling the cooling medium 5.
In operation, the food cooling medium 5 is delivered by the circulation fan 14 through the cooling device 21 provided in the upper chamber 8 and then is discharged vertically downward into the product chilling chamber 6 via either one or more openings in the chamber ceiling 12 or one or more discharge cones 23 extending a short distance downwardly into the product chilling chamber 6 above the elevation of the upper end 13 of the conveyor 10. Within the cooling chamber 6, the circulation fan 14 operates to draw the cooling medium 5 inwardly across the 360° spiral flites 18 for convective cooling and then into a vertical return flow path 20 which is surrounded by the spiral conveyor 10.
The spiral conveyor 10 can convey the food product 16 either upwardly or downwardly in the chilling chamber 6 during the cooling or freezing process but will typically convey the food product 16 upwardly. The spiral conveyor 10 has an axis of rotation (i.e., an axis of spiral travel) 24, typically also corresponding to the longitudinal axis of the vertical return flow passageway 20, around which the food product 16 is conveyed as it is contacted by the cooling medium 5 during the cooling process.
In another type of prior art spiral chiller, a plenum is provided outside of one or two side walls of the product chilling chamber to draw the cooling medium out of the product chilling chamber. Typically, the side wall of the product chilling chamber will have openings therethrough and will constitute the inner wall of the plenum. The chiller circulation fan(s) will also typically be located inside the plenum.
Heretofore, spiral chillers have had significant shortcomings which have significantly limited the types of chilling operations which can be performed in these chillers and have resulted in (a) significantly non-uniform flow, temperature, and chilling conditions within the product chilling chamber 6 and (b) non-uniform chilling results which vary, for example, depending upon whether the food product is conveyed near the inside edge 28, near the outside edge 26, or on the central portion of the spiral conveyor belt 10.
As seen in FIG. 1, the circulation fan 14 of the prior art system 2 operates to draw the cooling medium return flow 27 upwardly through the center of the cooking chamber ceiling 12 and then operates to discharge the cold cooling medium 5 vertically downward into the uppermost end of the product chilling chamber 6 through the one or more ceiling openings or cones 23. As will be apparent, the path of least resistance (i.e., the path of lowest pressure drop) is for the cooling medium to simply flow directly through the upper portion of the product chilling chamber 6 from the ceiling opening(s) or cone(s) 23 to the circulation fan 14. Consequently, flow rates and temperatures in the lower regions of the product chilling chamber 6 can be significantly lower than the flow rates and temperatures experienced in the upper regions of the product chilling chamber 6. In addition, temperatures in the lower regions can be significantly less cool than in the upper regions.
Also, as would be expected with a circulation system of this nature, dead zones are commonly created at various locations within the product chilling chamber 6 where the cooling medium 5 does not freely circulate. Further, the flow patterns which occur across the belt from the outside edge 26 to inside edge 28 can result in the food products placed toward the interior edge 28 of the belt receiving less beneficial heat transfer than the products placed toward the outer edge 26 of the belt, especially in the lower regions of the product chilling chamber 6 where lower cooling medium flow rates are experienced. This, in turn, can result in significant differences in cooling uniformity and product appearance.
The limited and non-uniform cooling flow circulation systems and patterns provided by the prior art spiral chiller 2 also result in slower cooling and freezing times and reduced product throughput. Because the cooling medium flow rate through the lower regions of the product chilling chamber 6 is significantly less than the flow rate in the upper regions of the product chilling chamber 6, much less cooling occurs in the lower regions than in the upper regions. Consequently, the conveyor rate must be slowed so that the product 16 is allowed to spend more time in the upper regions of the product chilling chamber 6 than would otherwise be required if the same flow rates and temperatures were experienced in the lower regions of the chiller. Further, the conveyor throughput rate must also be slowed to ensure that the product placed closest to the inner edge 28 of the spiral conveyor belt 10 is adequately chilled.
In addition, the available cooling and freezing applications for the prior art spiral chiller 2 have also been further significantly restricted because the prior art spiral chiller 2 is limited to the lateral flow of the cooling medium across the spiral flites 18 of the chiller 2. Heretofore, for example, to obtain any impingement effects which might be beneficial for crust freezing and other applications, it has been necessary to use one or more additional impingement chambers which impinge cold air or other cooling medium onto the product 16 before it enters or after it leaves the spiral chiller 2.